1. Field of the Invention
The present invention relates to an antenna array; and more particularly to an improved beam space antenna array method and system that has certain fully adaptive properties with respect to the discrimination of interfering signals at all angular locations relative to such array.
2. Description of the Prior Art
Antenna arrays that are effective to minimize each of a plurality of undesired signals that correspond to the number of elements in the array at all angular locations relative to the field of view of such array, is referred to as a fully adaptive antenna array. The antenna elements of such arrays may be dipoles, slots, or other conventional elements depending on the desired application. In constructing such fully adaptive antenna arrays, an adaptive circuit which functions to pass a plane wave signal received in the main beam of the array and to discriminate against strong interference in a minor lobe, sometimes referred to as a sidelobe is connected to each individual element of the array.
Further, fully adaptive antenna arrays, may be configured as beam space systems wherein each of the antenna input feed ports responds to a signal source in a predetermined angular direction relative to the array. This is accomplished by utilizing a phase shifting device, preferably a Butler Matrix that has a plurality of input ports and output ports. Each of the output ports is coupled to a respective antenna element, and the phase shifting device generates an orthogonal set of beams, each responding to a particular input of the Butler Matrix. One of the beams is regarded as the main beam; and the other beams are adaptively weighted by a multiple sidelobe canceller to form a cancellation beam which is subtracted from the main beam. Each of the other beams that feed the multiple sidelobe canceller has a null in the direction of look of the main beam; and therefore, the output has a constant response to the direction of look of the main beam.
The beam space fully adaptive array is particularly advantageous in that it discriminates effectively against interfering noise sources in the near in side lobe regions, which is the region in the side lobes near the main beam of the antenna pattern as well as in the far side lobe regions, which are those regions farthest from the main beam of the antenna. Also, the beam space fully adaptive antenna array permits faster convergence in the weighting of signals as compared to the fully adaptive array that is not of the beam space type when using gradient search algorithms.
However, such fully adaptive arrays require extensive hardware in their implementation, in that an adaptive circuit is required for each of the individual antenna elements.
Thus, because of these hardware requirements, a fully adaptive array with a large number of individual antenna elements is expensive, and relatively impractical, particularly for airborne radar. For example, to construct a fully adaptive array of 100 individual antenna elements, it is necessary to process the signals from each of the 100 antenna elements for each of 100 angular locations.
In order to minimize the extensive hardware required for such fully adaptive antenna arrays, partially adaptive beam space systems are utilized wherein a small number of the total antenna elements are selected at random, with adaptive circuitry provided for each of the selected elements. Such antenna systems are capable of discriminating against only that number of signal sources that correspond to the selected antenna elements; and may not fully be effective for discriminating against an undesired signal emanating from all of the angular locations in the field of view of the antenna array. Such partially adaptive antenna arrays appear to function effectively for the purposes for which they were intended; but tend to be prone to space ambiguity of the signal, and except when constructed as a beam space system, tend toward slow convergence of the gradient search types of weighting solutions, and have a relatively low signal to noise ratio.
To amplify the preceding discussion, and form a more detailed understanding of the state-of-the-art as it relates to adaptive arrays, reference is made to the following publications by way of example:
In the IEEE Transaction on Antennas and Propagation, Vol. AP-24 No. 5 September 1976, the articles entitled:
(1) "Adaptive Arrays With Main Beam Constraints" commencing at Page 650;
(2) "Adaptive Arrays" commencing at Page 585;
(3) In the proceedings of the IEEE Vol. 55, No. 12 December 1963, an article entitled "Adaptive Antenna Systems" commencing at Page 2143;
(4) In IEEE Transaction on Aerospace and Electronic Systems, Vol. AES-14, No. 1, January 1978, on article entitled "An Improved Algorithm For Adaptive Processing" commencing at Page 172;
(5) In IEEE Transaction on Aerospace and Electronic Systems July 1971, on article entitled "Effect of Envelope Limiting In Adaptive Array Control Loops" commencing on Page 698;
(6) Proceedings of IEEE Vol. 63, No. 12, December 1975, an article "Adaptive Noise Cancelling: Principles and Applications" commencing at Page 1692.
Thus, in accordance with the foregoing, it is desirable to provide a beam space antenna system and method that discriminates against interfering signals at all angular locations in the antenna's field of view and is particularly effective in discriminating against interfering noise sources in the near in side lobe regions, without the extensive hardware requirements of a fully adaptive array.